Great Circle Distance

distance.c

#include <math.h>
#include <stdio.h>

/**
 * @brief haversineDistance finds the
 * [great-circle distance](https://en.wikipedia.org/wiki/Great-circle_distance)
 * between two points on a sphere.
 * @see https://en.wikipedia.org/wiki/Haversine_formula.
 *
 * Parameters are the latitude and longitude of the first and second point in
 * radians, respectively.
 *
 * @return the great-circle distance in kilometers.
 */
 
double haversineDistance(double th1, double ph1, double th2, double ph2) {
    double dx, dy, dz;
    ph1 -= ph2;

    dz = sin(th1) - sin(th2);
    dx = cos(ph1) * cos(th1) - cos(th2);
    dy = sin(ph1) * cos(th1);
    return asin(sqrt(dx * dx + dy * dy + dz * dz) / 2) * 2 * R;
}

double to_radians(double degree) {
    return (degree * M_PI) / 180.0;
}

int main(int argc, char **argc){
  return 0;
}

Great Circle Distance formula, it is used to find distance between two point in a sphere.